1. Field of the Invention
This invention relates to transmitting a second signal with a video signal and more particularly to transmitting two signals with a video signal.
2. Description of the Prior Art
It has long been known that the video spectrum in common commercial formats such as NTSC, PAL and SECAM is not efficiently used. Although some different formats have been proposed for more efficient use of that spectrum, most of these formats such as the Japanese HDTV format cannot be received by conventional television receivers.
One method of more efficiently using the spectrum of a video signal is to time multiplex the video signal with a data signal. This is the approach used for transmitting close captioned data with the video signal. In particular, the close captioned data is transmitted during the vertical blanking interval. Specially designed receivers may receive and display the close captioned information while more commonly available commercial receivers may display the transmitted video signal without perceptible interference from the closed caption data. However, a drawback of close captioned television transmission data is that the data rate is low.
Other schema for more efficient use of the video spectra include use of the vestigial sideband of the luminance signal. An example of this is shown in U.S. Pat. No. 4,958,230. In that patent, a comb filter is applied to the vestigial sideband of the luminance signal at the intermediate frequency. A second, luminance signal is then frequency interleaved in that vestigial sideband.
A drawback of such approaches is that the phase and delay distortion of the filtering may adversely effect the blanking interval and the data transmitted during those intervals such as close caption data. The comb filter described in U.S. Pat. No. 4,958,230 may cause intersymbol interference or the like, thereby adversely affecting reception of close captioned data.
In addition, such filtering may also adversely effect the beginning and ending of the vertical and horizontal blanking intervals. Although these effects may not cause a perceptible degradation of the received signal, the delays and phase distortion may be perceptible during the transmission of test patterns. Still further, such effects may result in an adverse impact on testing of the transmitted signal.
Such adverse effects can be particularly adverse in testing the quality of various transmission links used for transmitting video signals such as coaxial and fiber optic cable links, microwave relay links, and satellite links. The presence of such delays and phase distortion makes it difficult for test equipment and technicians to discern whether the effects are caused by the filters used for transmitting the additional signal or by the transmission link.
Therefore, it is a first object of this invention to provide a system for more efficient use of the video spectrum. It is a second object of this invention to provide for more efficient use of the spectrum without interfering with the use of the transmission of the underlying video signal or other signals such as close captioned data transmitted during the blanking intervals. It is yet a further object of the invention to permit the testing of video transmission links.